Add __import__

[bswck]

I imagine a script illustrating __import__ should:

• show how all import X/import X.Y/from X import Y (and other variants) work under the hood with __import__
• highlight that __import__ != importlib.import_module
• mention importlib._bootstrap.__import__
• present the caching mechanism of modules
• optionally show module lock acquires/releases
• whatever else you suggest

[bswck]

Ideally, it could somehow be fluently transitioned into an overview of __package__, __path__ (and other module-level globals) and the entire import system (with mentions of sys.meta_path, sys.path, sys.path_hooks)